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Nucleons in a Bunch

The world of the very small is impossible to observe in complete detail. In the everyday world, once the billiard ball is struck, we can predict the final configuration on the pool table. This is because the method we use to observe the initial positions and motion of the balls – vision – doesn’t change appreciably those positions and motions. In the microscopic world described by quantum mechanics, however, Heisenberg’s uncertainty principle tells us that we can’t measure with arbitrary accuracy both position and velocity.

A similar principle affects the theory of quantum mechanical rotations. In principle, a rotating body has a total angular momentum (its propensity to keep spinning) and an orientation of the angular momentum in space. Since we have three spatial directions in our reality, there are three components of angular momentum. However, quantum mechanical theory tells us that we can know the total angular momentum, but any attempt to measure one of its components will disrupt the values of the other two components.

This leads to some confusion in interpreting the theory, even among physicists. The leader of my Ph.D. thesis project, hearing that I was doing well in my advanced coursework on quantum mechanics, expressed his confusion regarding the underlying physics of the system we were studying (muons in a magnetic field). I explained to him that the other two components still existed and influenced the time-evolution of the muon, but at the end only a single component could be measured.

This was a man that intimidated his collaborators with his brilliance and drive, and no one had ever clarified for him the basics of the quantum theory of angular momentum. This is not uncommon – often the words used to describe quantum processes are not reflective of the underlying mathematics of the theory. This allows lots of room for physicists to overplay the significance of their measurements.

Today we have a report from an experimental study that confirms that some quantum objects are not symmetric. This is not surprising, in some sense. The system, the nucleus of the barium atom, is a swirling stew of 56 protons and 88 neutrons. What the study reveals is that some number of these particles can clump together in a particularly ordered fashion. Once they achieve that configuration, the remaining protons and neutrons can’t push their way into the structure, and end up hanging like a barnacle on the outside.

Here’s a way of visualizing this: let’s say that we have twelve of those little magnetic balls. We can organize eleven of them into a nice little tetrahedron. But the twelfth ball is going to be stuck on the outside of the tetrahedron like a barnacle. It is going to ruin the regularity (what physicists call symmetry) of the assembly.

Why is this loss of symmetry exciting? Well, it seems to be a pretty natural consequence of self-organizing aggregates. But it’s also related to some principles used to guide the development of quantum mechanical theories. Remember, we can’t see this world very clearly, and touching its inhabitants disrupts their behavior. So to guide the development of theory, physicists have come up with abstract mathematical principles. Three important ones are charge (C), parity (P) and time (T) inversions. These state, respectively, that the equations that describe the quantum world should not change if:

particles are replaced with anti-particles

the particles are observed in a mirror, and

the universe is run backwards.

In actuality, it’s hard to create theories that violate all of these principles simultaneously (what is called CPT violation). However, the weak force that controls radioactivity is known to violate parity (P), though invariance is restored under CP.

So what is the significance of the asymmetry of Barium-144? The authors claim that it is parity violation in the strong and electromagnetic forces. The claim is based upon the observation that when looked at in the mirror, the barium atom will have its bump on the opposite side.

But that is not what parity violation means! The mirror-image barium nucleus is still allowed under the equations that describe its structure. In fact, it can also be obtained simple by walking around to observe it from the other side. That is certainly allowed in the theory.

We can contrast this with parity violation in neutrinos. Neutrinos, which only participate in the weak interactions, always have their angular momentum aligned against their direction of motion. They are “left-handed.” Observed in a mirror, however, that orientation changes: the direction of motion is reversed, but not the angular momentum. Thus the neutrino becomes “right-handed,” which is not known in nature, and so the equations of the weak interaction are violated by parity inversion. However, by adding charge inversion, the violation is removed: anti-neutrinos are indeed right-handed.

So in this case I’m afraid that got those making so much of the Barium-144 asymmetry have gotten their “nucleons in a bunch” for no good reason.

In general, the obscurity of quantum phenomena are not even well understood by physicists themselves. When they trumpet a great discovery, then, you should always ask yourself whether the practical implications of their work merit continued support by the public.

Unless, of course, you think of science as a cultural investment, like art or politics.